DT-340: Warn the user if they are leaving a story edit page with unsaved changes

[torresga]

Jira Ticket

https://ombulabs.atlassian.net/browse/DT-340

Motivation / Context

Currently if you leave the current page, you can lose information that was not saved. This feature warns you if you are leaving without saving information.

QA / Testing Instructions

1. Go to the Edit Story page.
2. Edit the story.
3. Try to navigate off the page by reloading, pressing the back button in the browser, clicking the logo or the Back button on the page.
4. Ensure that you get a popup asking you if are sure you want to reload/go back.

Screenshots:

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